This invention relates to transmission and processing of electrical signals using an optical carrier. More particularly, it relates to a fiber optic modulator system for modulating high power optical carriers to increase the optical power from the modulator without exceeding the optical power damage threshold imposed by the modulator.
The low loss and large intrinsic bandwidth of fiber-optic links make them invaluable for the transmission of information at RF frequencies. In most photonic links, the information is converted from RF signals to optical signals through the use of an external Mach-Zender modulator (MZM). Most electro-optic modulators (EOMs) are based on proton exchanged or Ti-interdiffused waveguides in LiNbO3. When an electric field is applied across a waveguide, the optical path length of the waveguide is altered, allowing the phase of the output signal to be controlled. This effect is used to both alter the phase of the light (phase modulators) and to produce amplitude modulation when the waveguide is placed within an interferometer. However, there is a limitation on the amount of light that may be passed through a LiNbO3 waveguide. This limitation is based on the photorefractive effect, which optically damages LiNbO3. The material is not physically damaged, but the high optical fields cause charge migration in the LiNbO3, which in turn alters the refractive index of the material, thus reducing the output. Typical values for the optical damage thresholds of Ti:LiNbO3 and proton exchanged LiNbO3 are approximately 10 MW/cm2 and 40 MW/cm2, respectively. These threshold levels limit the amount of optical power that may be inserted into a modulator to about 50-100 mW for Ti: LiNbO3 modulators and to about 200-400 mW for proton exchange based LiNbO3 modulators, depending upon the dimensions of the waveguide. Thus, there is a need to enhance the performance of fiber optic links by providing an intrinsic electronic gain, low noise, and higher optical power.
In one approach as illustrated in FIG. 1, a continuous wave optical signal from a source 12 is modulated by an electro-optic modulator (EOM) 16, which is preferably a LiNbO3 amplitude modulator (MZM), and detected by a high-frequency detector 18. The optical power transmitted through the MZM 16 is limited by its damage threshold, or by compression in the detector 18. Thus, there is a need to overcome the problems encountered by the prior art by improving the analog modulation of a continuous wave (CW) optical signal modulated by an MZM.
Accordingly, the present invention provides an apparatus and method to increase the output power from LiNbO3 MZM style optical intensity modulators without exceeding the optical power damage threshold imposed by the modulator. In the exemplary embodiment of the present invention, the fiber optic modulator apparatus of the present invention includes: at least two polarization maintaining (PM) couplers, a piezoelectric transducer (PZT) cylinder, at least one phase modulator. Signals from an optical source are split into two paths by a PM coupler. The two paths form a Mach-Zender Modulator (MZM) with a LiNbO3 phase modulator in one path and a fiber looped PZT in the other. The LiNbO3 phase modulator imprints an RF signal onto one path of the MZM cavity, while the fiber wrapped PZT is used to control the path length difference between the two MZM paths. The two optical paths are recombined in a second PM coupler. The second PM coupler and a 1-2% coupler (also referred to as fiber tap) are used to sample a small portion of the MZM output signal which is fed back to a phase locked loop (PLL) circuit for providing feedback voltage to the fiber wrapped PZT in the second arm of the MZM in order to ensure the phase of the signals in the two arms of the MZM are matched to within a fraction of the laser linewidth. The output power of the modulator is improved by using a LiNbO3 modulator within a fiber Mach-Zender cavity.
In another embodiment, a polarization maintaining Erbium Doped Fiber Amplifier (EDFA) is inserted into the MZM cavity of the fiber optical link between the LiNbO3 phase modulator and the PM coupler of the first optical path.
In yet another embodiment, a second phase modulator is inserted into the second arm of the fiber MZM to allow for dual-drive modulation or adjustable-chirp modulation.
In one aspect, the present invention provides a fiber optic modulator system, comprising: an optical source; a first polarization maintaining (PM) coupler for splitting a signal received from the source into two optical paths, the two paths forming a Mach Zender Modulator (MZM); a phase modulator disposed in a first optical path; a piezo-electric transducer (PZT) disposed in a second optical path; a second PM coupler for recombining the first and second paths; and a detector for detecting the output of the MZM. The system of claim 1 further includes a fiber tap for sampling a portion of the MZM output; a d.c. photodetector for detecting the output of the fiber tap; and phase locked loop (PLL) for receiving a signal from said d.c. photodetector and providing a feedback signal to said PZT, thus allowing the relative phases of said first and second paths to be controlled. The PZT preferably controls the optical path length of the second optical path. The phase modulator is preferably of lithium niobate (LiNbO3). The phase modulator is used to imprint an analog input signal into the first path, the analog input signal modulating a signal from the optical source. The phase modulator enables the phase of input signals to be modulated by an RF signal, the phase modulation is detected by the second PM coupler resulting in constructive and destructive interference. The phase modulator is preferably of the type that maintains the optical polarization of signals from the optical source which may be a diode pumped Nd:YAG ring cavity laser. The fiber modulator system further comprises a polarization maintaining erbium doped fiber amplifier disposed between the phase modulator and the second PM coupler, a second phase modulator disposed in said second path.
In another aspect, the present invention provides, in a fiber optic communication system having a plurality of fiber optic modulators and a plurality of fiber optic links, a method of enhancing the performance of fiber optic links comprising the steps of: providing an optical source; providing a first polarization maintaining (PM) coupler for splitting signals from the optical source into first and second paths, the first and second paths forming a Mach-Zender Modulator (MZM) cavity; disposing a phase modulator in the first path; disposing a piezo-electric transducer (PZT) in the second path; disposing a second PM coupler for combining the outputs of the phase modulator and the PZT; and detecting the output of the second PM coupler. The method further comprises providing a fiber tap for sampling a portion of the MZM output; providing a d.c. photodetector for detecting the output of the fiber tap; and providing a phase locked loop (PLL) for receiving a signal from said d.c. photodetector and providing a feedback signal to the PZT, thereby allowing the relative phases of the first and second paths to be controlled. The method further comprises the steps of imprinting an analog RF signal onto the first path using the phase modulator; and controlling the length of the second optical path using the PZT, and disposing a second phase modulator in the second path to allow for dual drive modulation. The output of the second PM coupler is detected using a plurality of high frequency photodetectors. The outputs of the high frequency photodetectors may be subtracted to implement a balanced photodetection scheme.
In another aspect, the present invention provides a fiber optic link system for transmitting signals from a source to a destination having a fiber optic modulator system, the fiber optic modulator system comprising: an optical source; a first phase modulator (PM) coupler for splitting a signal received from the source into two optical paths, said two paths forming a Mach Zender Modulator (MZM);a phase modulator disposed in a first optical path; a piezo-electric transducer (PZT) disposed in a second optical path; a second PM coupler for recombining the first and second paths; and a detector for detecting the output of the MZM according to various embodiments of the invention.
It will thus be seen that according to the present invention a high power fiber optic modulator is provided to overcome the problems faced by the prior art. While the invention has been herein shown and described in what is presently conceived to be the most practical and preferred embodiment, it will be apparent to those of ordinary skill in the art that many modifications may be made thereof within the scope of the invention, which scope is to be accorded the broadest interpretation of the appended claims so as to encompass all equivalent methods and apparatus.